The present invention relates to a method of heating natural gas in preparation for a reduction in its pressure. In a natural gas distribution system, the gas generally travels from the field at relatively high pressure and velocity. Prior to introduction into the system that will deliver the gas to the end consumer, a pressure reduction must be accomplished. Pressure reduction stations, generally known as City Gate stations, are usually provided to perform this function. The City Gate stations use pressure-reducing valves, commonly known as JT valves, to reduce the gas pressure to a desired level.
In colder climates, the natural gas entering the City Gate station must undergo a heating process. Although the main supply lines entering a City Gate station are generally buried underground, they typically are not deep enough to be fully insulated from the effects of the ambient air temperature. Thus, in cold climates the temperature of the gas in the supply lines may approach freezing. Also, a natural result of the gas pressure reduction is a decrease in the gas temperature. This phenomenon is known as the Joule-Thompson effect. The larger the pressure reduction, the larger the decrease in gas temperature. The large pressure reductions generally performed in a City Gate station coupled with the often low temperature of the incoming natural gas may frequently result in a gas temperature, after pressure reduction, of below freezing. Such a temperature may be sufficient to freeze the small amounts of water that commonly travel with the natural gas, and also to cause a freezing of the JT valves. Thus, one aspect of the present invention is to employ a method for maintaining the temperature of the natural gas after pressure reduction at a level sufficient to prevent freezing.
In a typical City Gate station, the natural gas is generally heated by some method prior to entering the JT valves. The amount of heating required will depend on the pressure reduction necessary, and the temperature of the incoming natural gas. The amount of energy required for heating the natural gas may be substantial, especially in colder climates. In such a heating system, the natural gas itself is typically utilized as the energy source for generating the required heat.
The present invention provides a method for heating natural gas in a City Gate Station without requiring the burning of natural gas for producing the required heat. In a preferred embodiment of the invention, the system comprises a heat exchanger coupled to an air compressor that is powered by an air motor. The incoming natural gas, already naturally under pressure, is circulated through the heat exchanger where it picks up heat. The warmed gas exits the heat exchanger where it is divided into at least two paths. The majority of the gas passes directly to the JT valves, while a smaller portion travels through a reduced diameter conduit to the air motor. Because the warmed gas has a relatively high pressure and velocity, the air motor is able to generate enough power to drive the air compressor.
In one preferred embodiment of the present invention, an air cooled air compressor is preferably employed. The heated air stream leaving the air compressor is circulated through the heat exchanger to warm the heat exchanger's working fluid. Heat may be imparted to the compressed air stream by drawing heat from the air compressor itself, and the volumetric flow rate of the heated air may be increased by employing a device such as an air amplifier.
In another embodiment, a liquid-cooled air compressor is preferably used. However, instead of using the compressed air stream from the air compressor to warm the heat exchanger's working fluid as in the previous embodiment, the compressed air stream in the present embodiment is used to drive a second air motor. The second air motor powers a pump which circulates hot coolant from a portion of the air compressor to the heat exchanger. After circulating through the heat exchanger, the reduced temperature coolant is returned to the air compressor.
In a third embodiment, an oil-flooded rotary compressor is preferably utilized. The rotary compressor pumps a combined stream of heated oil and compressed air to the heat exchanger, where it is used to increase the temperature of the heat exchanger's working fluid. The oil and compressed air stream cools as it circulates through the heat exchanger, and then returns to the rotary compressor. The rotary compressor contains an oil separator which then separates the oil from the air.